1. Field of the Invention
The present invention relates to methods and formulations for the therapy of cystic fibrosis, Bartter's syndrome and secretory diarrheas such as cholera, and for diuretic treatment.
2. Discussion of the Background
Cystic fibrosis: This is a congenital disease for which an accurate diagnosis has long been available. Historically, a midwife would lick the forehead of a newborn. If the sweat tasted abnormally salty, the infant was destined to die of pulmonary congestion and its side effects. Today, cystic fibrosis remains the most common lethal congenital disease among caucasians where it has a prevalence of about of about 1 in 2,000 live births.
Cystic fibrosis is a disease of secretory epithelia, tissues that mediate the transport of water, salt, and other solutes between the blood and the outside world. Epithelial cells exhibit anatomical and functional polarity. The basolateral membrane, which faces the blood, and the apical membrane, which faces the lumen (the outside world) mediate different transport events. Together they give rise to net chloride transport across the epithelium from blood to lumen.
Sodium and water accompany the transport of chloride, resulting in secretion of a solution of sodium chloride into the lumen. The secretion requires activation of the secretory pathways by hormones and neurotransmitters, which utilize the intracellular second messengers adenosine 3',5'-monophosphate (cyclic AMP) or calcium.
Although the survival of cystic fibrosis has improved in recent years, the median survival is still only about 25 to 30 years despite intensive supportive and prophylactic treatment. There is thus a clear need for new therapeutic approaches to this uniformly fatal disease in which, all patients suffering from the disease develop chronic progressive disease of the respiratory system. The most common cause of death is pulmonary disease. Also, in the majority of cystic fibrosis patients, pancreatic dysfunction occurs, with hepatobiliary and genitourinary diseases being also frequent.
The clinical manifestations of the disease appear to be consequences of the secretory defect with diminished secretions producing obstruction of airways and pancreatic output. Current therapeutic approaches are designed to improve air flow indirectly by bronchodilatation, reduction of inflammation with glucocorticoids and control of superimposed infections with antibiotics. Most recently, direct attempts to improve quality of secretions have been made utilizing aerosolized amiloride (U.S. Pat. No. 4,501,729) or 9-ethyl-6,9-dihydro-4,6-dioxo-10-propyl-4H-pyrano(3,2-g)quinoline-2,8-dica rboxylic acid or a pharmaceutically acceptable derivative thereof (U.S. Pat. No. 4,866,072).
Amiloride aerosol appears to slow the decline in pulmonary function associated with cystic fibrosis. These beneficial effects are attributed to increased clearance of secretions, presumably due to reduced sodium absorption. This treatment however does not address the primary abnormality of the disease.
Recent studies have provided new information concerning the abnormalities in cellular transport which underlie the secretory defect in cystic fibrosis. There is now direct evidence indicating that the chloride channel in cystic fibrosis airway epithelia does not activate in response to cAMP as it does in normal airway cells. At present, although direct exogenous activators of the chloride conductance have not been identified or characterized, the development of such compounds should prove to be extremely beneficial, either alone or in combination with alteration of sodium transport.
Bartter's syndrome: In 1962 Bartter described a syndrome consisting of hypokalemia due to renal potassium wasting, elevated plasma renin activity, and aldosterone secretion, normotension, hyporesponsiveness of blood pressure to infused angiotensin II and hyperplasia of the granular cells of the juxtaglomerular apparatus. Weakness, or periodic paralysis, and polyuria occurred because of chronic potassium depletion. Hyperplasia of renal medullary interstitial cells, which produce prostaglandins (PG) E and F, has been described in some patients, along with very elevated PGE.sub.2 production. Inheritance is autosomal recessive. Manifestations commonly begin in childhood.
While the pathogenetic sequence of this disease has not been proved with certainty, depressed reabsorption of certain chloride in the proximal tubule or thick ascending limb of the loop of Henle is thought to be the basic underlying defect in this syndrome. The exact basis for the tubular reabsorptive defect is uncertain, but it may be part of a widespread inherited disorder.
Excessive sodium chloride losses in the urine causes depletion of extracellular fluid volume, which in turn stimulates hyperplasia of the granular cells of the juxtaglomerular apparatus and increases renin production. The resulting increase in angiotensin II production stimulates aldosterone secretion. Renal tubular defect permits continued delivery of sodium to the sodium-potassium exchange sites in the distal nephron, which are stimulated by a supernormal concentration of serum aldosterone, and renal wasting of potassium ensues. Blood pressure remains normal because the pressure effects of angiotensin II are offset by depletion of extracellular volume. Infusion of angiotensin II produces less than the usual rise in arterial pressure because angiotensin II levels are already elevated.
With patients suffering from Bartter's syndrome, the dietary intake of sodium chloride and potassium is liberal. Often potassium supplements are required despite a high dietary intake. Pharmacological blockage of aldosterone effects on distal tubules with spironolactone can prevent potassium wasting, though sodium intake must be increased. Inhibition of prostaglandin synthesis with indomethacin has met with varying success. Beta adrenergic blockade has also been used, with some success, to lower renin production. But there is a clear need for new therapies directed at stimulating sodium chloride reabsorption in the kidney.
Secretory diarrheas: Cholera is a secretory diarrhea caused by infection with vibrio cholera organisms. The organisms produce an exotoxin which causes the cells lining the intestine to secrete sodium, chloride and water. The massive secretion causes loss of extracellular fluid volume leading to lowering of blood pressure, shock and death if not appropriately treated. Untreated the disease lasts for 2 to 7 days. Current therapeutic regimens include tetracycline to treat the organism and replacement of fluids parenterally and orally. The availability of an orally effective compound which would reduce secretion would have major therapeutic benefits in this disease.
Diuresis: Diuretics are drugs which increase the urinary excretion of sodium and chloride by interfering with normal transport processes at various sites along the nephron of the kidney. They are used in the treatment of disease with increased sodium, chloride and fluid retention such as congestive heart failure, hypertension, cirrhosis and nephrotic syndrome.
Potent diuretics usually work in the ascending limb of the loop of Henle where 20 to 30% of the filtered load is reabsorbed. These drugs include ethacrynic acid, furosemide and bumetanide. A major side effect of these drugs is loss of potassium in the urine with consequent lowering of potassium concentration in the blood (hypokalemia). This is due to increased potassium secretion into the urine by cells lining portions of the nephron distal to the loop of Henle in response to increased delivery of sodium from the loop of Henle. A potent diuretic which inhibits chloride (and sodium transport) in the loop of Henle and also blocks potassium secretion in more distal portions would be a significant improvement over currently available drugs.